1. Field of the Invention
The present invention generally concerns the fabrication of terminals on ceramic capacitors, and the terminals so fabricated.
The present invention particularly concerns (i) the fabrication of terminals on and to ceramic capacitors by process of plating where lateral growth of the plating electrically connects exposed electrodes, and more preferably by process of electroless plating (also known as electroless deposition or electrodeposition); and (ii) ceramic capacitor terminals having multiple plated or electrolessly plated (also known as electrolessly deposited) layers.
2. Background of the Invention    2.1 General Background
The capacitor is an electrical charge storage device and is one of the basic building blocks in electronics. Capacitors are made in many types including ceramic, tantalum, aluminum electrolytic, and film. The preferred embodiment of the terminal fabrication method of the present invention will be seen to be specific to ceramic capacitors. However, the terminal fabrication method of the present invention will be seen to be extendable to other electronic components, such as inductors, other than just capacitors.
Ceramic capacitors cover a wide range of applications including: charge storage, DC blocking, circuit components coupling, AC by-pass, and transient voltage suppression. See, e.g., Galliath, A. P., Novacap Technical Brochure, website <http:\www.novacap.com>, 2001. The challenge for the ceramic capacitor manufacturer is the same for any manufacturer: reduce costs and increase yields. Worldwide production of ceramic capacitors range in many billions of capacitors annually. See Electronic Industry Association, Market Data Book, 1987. The selling price, circa 2002, on some smaller capacitors is on the order of 1000 capacitors per United States dollar. To remain competitive many manufacturers are turning to new technology to reduce costs.
Traditionally ceramic capacitors require the use of precious and semiprecious metals, typically palladium and silver for electrodes as well as end terminals. In the past five years, the cost of palladium has been extremely volatile. The cost per troy ounce has fluctuated from under than $200 to over $1000 U.S. See Palladium (NYMEX): Monthly Price Chart, website <http:\futures.tradingcharts.com/chart/PA/M> on the Internet circa 2002. This has led to a strong push in the development of ceramic materials that use base metal electrodes such as copper and nickel. See Mistler, R. E., Twiname, E. R., Tape Casting Theory and Practice, The American Ceramic Society, 2000. Likewise, there has been a strong push in process improvement and automation in order to improve yields and reduce fabrication time.
The present invention will be seen to concern the terminating of ceramic capacitors, a process where end terminals of metal are applied to ceramic bodies containing electrodes. The present, circa 2002, industrial method for realizing these electrodes uses thick film cermet pastes. See Galliath, A. P., Novacap Technical Brochure, website <http:\www.novacap.com>, 2001. More regarding this method will be discussed in section 2.8, below.    2.2 The Basic Capacitor
The basic capacitor is a charge storage device composed of an insulator sandwiched between two conductors, as shown in FIG. 1. See Galliath, A. P., Novacap Technical Brochure, website <http:\www.novacap.com>, 2001. The intrinsic properties of the dielectric material determine the characteristics of the capacitor. Some of these characteristics include capacitance, insulation resistance, and dielectric strength.
Capacitance is measured in Farads. A Farad is defined as one Coulomb per Volt. A Farad is a relatively large amount of charge, so it is more common to use units such as micro-Farad (μF), nano-Farad (nF), and pico-Farad (pF).
Dissipation factor, DF, is defined as the loss tangent in an AC circuit. See Harper, C. A., Handbook of Thick Film Hybrid Microelectronics, McGraw-Hill, New York, 1974. For simplicity, DF can be defined as the loss factor. The ideal capacitor, in which the dielectric would exhibit infinitely high resistance, would have a loss factor of zero. Thus once a charge has been applied the ideal cap would hold that charge without any loss. Two main factors contribute to the DF of ceramic capacitors. The first is an intrinsic property of the ceramic, dielectric absorption, and the second is the equivalent series resistance (ESR) associated with all the electrical connections involving the cap. Sources of ESR include resistivity of the electrodes, connections between the electrodes and terminals, and solder connections to printed circuit boards (PCBs). Connections between electrodes and terminals are of primary interest to the present invention. A capacitor has a dissipation factor, or DF; the equivalent circuit for which is well known. Dielectric absorption is represented as a resistor connected parallel to the capacitor, and an equivalent series resistance, or ESR, is represented as a resistor in series with the capacitor.
Insulation resistance (IR) is a measure of the dielectric material's ability to block DC current flow when a DC bias is applied. This relationship may be plotted, and IR is commonly used instead of R to differentiate between a capacitor and a resistor. For ceramic dielectrics, the IR is very high and is typically expressed in terms of 106 or 109 ohms. The IR for ceramic capacitors is typically measured at the rated working voltage, and the IR values can be on the order of 1012 ohms.
Dielectric strength is a measure of the ceramic's ability to withstand a high bias without electrical breakdown and is typically expressed in terms of volts per mil or volts per micron of thickness. If an applied voltage is steadily increased, then at a certain point the applied field will become be high enough to drive free electrons between the two conductor plates. The heat generated will then result in a breakdown of the dielectric layer. In ceramic capacitors, dielectric breakdown results in the catastrophic failure of the cap. The typical ceramic cap can exhibit dielectric strength on the order of 1000 volts per mil or 40 volts per micron.    2.3 Ceramic Capacitors
Ceramic capacitors are available in a wide array of capacitance values, working voltages, sizes, and for various applications. Capacitance values are available from less than one pico-Farad to hundreds of micro-Farads. There is no real maximum value since capacitance can be added by putting two or more capacitors in parallel. Working voltages are available from less than 10 VDC to over 10,000 VDC. Typical sizes range from size “0201” and up. The 0201 size code indicates a part that is nominally 0.020 inch long, 0.010 inch wide, and up to 0.010 inch thick. In many cases, manufacturers will make the thickness equal to the width in order to maximize capacitance and for ease of handling. Ceramic capacitors are available for various applications, and ceramic dielectrics have been formulated to meet these applications. Some dielectric formulations maximize capacitance while others are best suited for high voltage applications. Some formulations function best at cryogenic temperatures while others will perform at 200° C. or more. Due to their versatility, ceramic capacitors are found in virtually all electronic applications from household electronics to medical implants to space and military applications.    2.4 MLCC Construction
Multilayer ceramic capacitors (MLCCs) are manufactured by interleaving multiple layers of ceramic dielectric and metal electrodes, as shown in FIG. 2. The layer thickness (t), also referred to as electrode spacing, is generally proportional to the voltage rating of the cap. Depending on the voltage rating, the layer thickness can be less than 10 μm or much thicker for high voltage applications. Each layer of an MLCC is in effect a single capacitor. When the electrodes are electrically connected with a metallic end terminal then the result is the summation of the capacitance from all the individual layers.    2.5 End Terminals
The end terminals electrically connect together each of the two opposing sets of electrodes of the capacitor and serve as terminals for electrical connections to PCBs. The typical end terminal material is a thick film cermet paste, usually composed of either Ag powder or Pd—Ag powder and glass frit. The terminals are formed by dipping the MLCC into the thick film paste and sintering the paste in the range of 600° C.-800° C. This does not affect the ceramic chip since the unterminated MLCC is processed at 1100° C.-1300° C. Sintering causes the glass frit to adhere to the ceramic. The metal powder also forms a diffusion bond to the electrodes, thus making electrical connections to the metal electrodes.
The dipping process creates end terminals that wrap around all four sides of the capacitor. The wrap-around structure is necessary for good adhesion to the ceramic body. Terminal adhesion strength is typically higher than the tensile strength of the ceramic. Terminals with minimal wrap-arounds tend to have lower adhesion strength and would be susceptible to peeling.
Once terminated, a MLCC is typically electroplated with nickel and then tin or tin-lead solder in order to be surface-mountable. Surface-mounting is the soldering of components onto PCBs. See Prassad, R. P., Surface Mount Technology Principles and Practice, Van Nostrand Reinhold, New York, 1989. The nickel layer is typically referred to as the barrier layer. Although nickel is solderable it does not readily dissolve in molten solder as silver does. The nickel layer functions as a protective barrier for the silver end terminals when the capacitors are soldered to PCBs. Tin and tin-lead coatings serve to protect the nickel from oxidation and to make components readily solderable. FIG. 4 is a cross-sectional micrograph of a silver terminal that has been plated with nickel and tin-lead.    2.6 Plating of End Terminals
Typically, end terminals are electrolytically plated with a layer of nickel followed by a layer of tin, tin-lead, or gold. The traditional method to plate ceramic capacitors is barrel plating. Barrel plating is the process in which parts are placed in a rotating mesh basket, typically made of polypropolene, and immersed in a plating bath as shown in FIG. 6. See Singleton, R. Barrel Plating, Metal Finishing Guidebook and Directory, 2001, p. 340-359,    2.7 Improving MLCCs
As mentioned in Section 2.1 there is ongoing materials development in the ceramic capacitor industry in order to reduce costs. In the past 30 years over 500 patents relating to ceramic capacitors have been issued in the United States. These patents cover every aspect of manufacturing, and in the last 5 to 10 years much effort has been going towards creating ceramics that can be processed using inexpensive metals for electrodes.
The cermet method of forming end terminals described in Section 2.5 has been the industry standard for many years. There has been much improvement in quality of the terminals; however, the basic composition remains a paste of metal powder and glass frit. There has been little work on an alternative method to form MLCC terminals. About a dozen patents are related to end terminals. Three of these patents involved alternative methods terminating MLCCs. All three processes use cermet pastes.
Westwater showed that sputtered terminals would reduce board space. See Westwater, R., Sputtered Terminations Gain Space, Electronic Engineering, vol. 65, August 1993, p. 31.
Scrantom also patented a sputtering process of applying terminals to ceramic bodies. See Scrantom D. G., Hopkins L., Method of Applying Terminations to Ceramic Bodies, U.S. Pat. No. 4,561,954, 1985.
The importance of board space reduction will be discussed in Section 3.3. Sputtered terminals do help to reduce board space; however, it is likely that this method would make parts more costly to produce.
The present invention teaches forming terminals onto bare MLCCs using electrodeposition. In 1974 Hurley patented a method of terminating ceramic capacitors where a thin film of immersion gold is deposited onto the electrode edges prior to applying termination paste. See Hurley T. P., Multilayer Ceramic Capacitor and Method of Terminating, U.S. Pat. 3,809,973, 1974. The capacitors used in Hurley's method have base metal electrodes, and the gold film prevents oxidation of the electrodes during sintering in air.    2.8 Specific Prior Patents
Other United States patents of relevance to the present invention include the following.
U.S. Pat. No. 3,665,267 to Acello for CERAMIC CAPACITOR TERMINALS shows bond pads to a ceramic capacitor that are soldered. A monolithic multi-electrode capacitor chip has silver electrode pickups on opposed edges of the capacitor stack. A multi-metal clad strip is affixed on the silver pickup, therein affording a smooth compatible terminal surface for further bonding purposes as the capacitor is used in hybrid circuitry.
U.S. Pat. No. 4,246,625 to Prakash for a CERAMIC CAPACITOR WITH CO-FIRED END TERMINATIONS shows co-fired terminations. A ceramic body containing embedded metal electrodes is provided with end termination configurations using a paste containing base metal particles, glass frit and MnO2; the body and end terminations being co-fired to provide a ceramic capacitor.
U.S. Pat. No. 4,293,890 to Varsane for a CERAMIC CAPACITOR WITH END TERMINALS shows a leaded capacitor. A lead wire for a miniature capacitor having a U-shaped clamp at one end and being removably attached to a carrier, such as a sprocketed ribbon, at the other end. Each of the U-shaped clamps grasps and holds a terminal end of the capacitor. The carrier is used with conventional geared wheels and reels to move the capacitors and lead wires from station to station during their assembly procedure. When assembly is completed, the leads can be removed from the carrier.
U.S. Pat. No. 4,346,429 to DeMatos for a MULTILAYER CERAMIC CAPACITOR WITH FOIL TERMINAL shows a special style of terminal. Namely, a ceramic capacitor has a metal foil terminal strip configuration to reduce high frequency inductance.
U.S. Pat. No. 4,517,155 to Prakash, et al. for a COPPER BASE METAL TERMINATION FOR MULTILAYER CERAMIC CAPACITORS shows copper end terminations. These terminations—reportedly of excellent electrical and mechanical properties—are provided on multi-electrode ceramic capacitors by applying copper, glass frit metallizations to the ends of a ceramic capacitor and firing the applied metallization in an atmosphere of nitrogen which contains a controlled partial pressure of oxygen.
U.S. Pat. No. 4,561,954 to Scrantom, et al. for a METHOD OF APPLYING TERMINATIONS TO CERAMIC BODIES concerns sputtered terminals. A method of terminating a multilayer ceramic capacitors and like electronic components is disclosed. In accordance with the method the capacitors are loaded into apertures formed in an elastomeric mask such that only the surface portions to be metallized are exposed. In advance of loading, the surfaces of the mask are pre-coated, preferably by a sputtering procedure, so as to preclude “out-gassing” of the mask material during sputtering.
U.S. Pat. No. 4,571,276 to Akse for a METHOD FOR STRENGTHENING TERMINATIONS ON REDUCTION FIRED MULTILAYER CAPACITORS concerns the metallurgy of capacitor terminals. The strength of end terminations on multilayer capacitors employing base metal electrodes is increased by heating the terminations, subsequent to firing in a reducing atmosphere, in an atmosphere in which the oxygen partial pressure is at least equal to that of air for a period of at least 15 minutes at a temperature of 375° C.-600° C.
U.S. Pat. No. 4,757,423 to Franklin for a FUSE FOR ELECTRONIC COMPONENT does not concern a capacitor, but does describe how the technique of applying metal-coated polymer particles dispersed in a resin binder as a conductive paste, a variant upon the common method of creating a terminal for a ceramic capacitor, may be used for a fuse. In the Franklin patent a solid electrolytic capacitor has an anode body and an anode wire and lead out connections. In series with the connections and the body is a fusable link formed of a composite of low melting point conductive plastics metal matrix. The fusable link is in the form of a pad of this material. In a preferred form this material is made by compressing into sheets metal-coated polymer particles. The sheet is cut into pads and inserted into the capacitor assemblies to act as a combined thermal and electrical fuse. Preferably the pads are less than 1 mm thick and coated on both sides with solder and approximately 1 mm square. The pads can be reflow soldered between the anode and the lead frame or negative wire termination. Alternatively it can be reflow soldered between the anode wire and the positive wire termination. As the current reaches high level if a fault develops in the capacitor, the metal layer will melt and also melt the plastics. The metal will then disperse in the liquid plastics and on cooling will not re-establish conduction because it is no longer in the same physical form. Alternatively the fusible link comprises metal-coated polymer particles dispersed in a resin binder and applied as a conductive paste.
U.S. Pat. No. 4,806,159 to De Keyser, et al. for an ELECTRO-NICKEL PLATING ACTIVATOR COMPOSITION, AND METHOD FOR USING A CAPACITOR MADE THEREWITH shows a plating, and plating activator, composition. A plating activator composition that is largely silver is applied in a thin film to two surface areas of a ceramic chip capacitor. Subsequently, many such chip capacitors are electrolytically nickel plated, e.g. are electro-nickel barrel plated to provide two strongly adhered nickel terminals to the component. This activator composition consists essentially of at least 85% Ag, from 0.1 to 7% Pd, from 1% to 10% of an element selected from Cu, Si, Bi, Zn, Fe, Ni, Sn, Zr, Nb, Sb, Mn and combinations thereof. The terminals are alleged to be strong, truly conformal and are highly manufacturable.
U.S. Pat. No. 4,881,308 to McLaughlin, et al. for a METHOD OF TERMINATING LEAD FILLED CAPACITOR shows the use of lead in capacitor terminals. A method of manufacturing a ceramic capacitor of the lead filled type includes coating the ends of the ceramic monolith with a terminating paste incorporating oxidizable metal particles characterized in that the lead will not wet to oxides of the metals but will wet to un-oxidized or lightly oxidized increments of the metals. The paste is fused in an oxidizing environment or is fused in an inert environment and thereafter heated in an oxidizing environment with the result that the metal increments adjacent the exterior of the fused coating are oxidized whereas the metal at the interior portions of the paste are un-oxidized or only slightly oxidized. Upon metal injection, the lead will wet to the interior portions of the fused paste but will not wet to the exterior of the paste whereby injected chips may be readily separated and whereby the size of the chip is rendered predictable due to the absence of adherent lead.
U.S. Pat. No. 5,363,271 to Pepin for THERMAL SHOCK CRACKING RESISTANT MULTILAYER CERAMIC CAPACITOR TERMINATION COMPOSITIONS describes a termination paste. A thick film conductor composition suitable for use in forming terminations for titanate-based MLCs comprises finely divided particles of: (a) electrically conductive precious metal, and (b) metal oxide-based glass having a Dilatometer softening point of 400° C.-700° C. The (b) metal-based oxide glass preferably consists of at least one glass modifier having an ionic field strength higher than the ionic field strength of the titanate cation, both (a) and (b) being dispersed in an organic medium.
U.S. Pat. No. 5,670,089 to Oba, et al., for a CONDUCTIVE PASTE FOR MLC TERMINATION concerns the use of conductive paste in the terminals of a multilayer capacitor (MLC). The purpose of the invention is to provide a terminal electrode composition for a multiple-layered capacitor that is suitable for a plating base and that has improved resistance to heat stress as the result of sintering at a low temperature (high reliability). The terminal electrode composition particularly for a multiple-layered capacitor of this invention is made of precious metal particles and 0.5-7 wt. % (based on the weight-of the precious metal particles) of an inorganic binder having a 400° C.-500° C. glass transition point and a 400° C.-550° C. glass softening point.